The present invention, in some embodiments thereof, relates to modification of the surface properties of a material, and, more particularly, but not exclusively, to methods of decreasing the contact angle of a liquid surface, to liquid surfaces featuring decreased contact angle and to uses thereof.
Control of surface properties is beneficial in a variety of fields such as metallurgy, electronics, medicine and applied science. One such property is surface wettability.
Wettability accounts for a surface's ability to interact with a liquid, typically an aqueous liquid such as water. A surface is said to be wetted if a liquid spreads over the surface evenly without the formation of droplets. When that liquid is water and it spreads over the surface without forming droplets, the surface is said to be hydrophilic. Practically, hydrophobicity and hydrophilicity are invertly-relative terms.
The utility of hydrophilic or wetted surfaces varies widely. Anti-fog coatings exploit high surface energies to flatten water droplets rather than allowing them to form light-scattering droplets. In biological systems, hydrophilic surfaces can reduce nonspecific bonding of proteins. Hydrophilic surfaces allow formation of tightly adherent layers of water with high lubricity which may be used, for example, to disperse particles in aqueous coatings and oil-in-water emulsions.
Water/oil systems exhibit low contact hysteresis, and the investigation of wetting regimes of such systems has been extensively explored in recent years [Shillingford et al., Nanotechnology 2014, 25:014019; Wong et al., Nature 2011, 477:443-447; Grinthal & Aizenberg, Chem Mater 2014, 26:608-708; Nosnoysky, Nature 2001, 477:412-413; Eifert et al., Adv Mater Interfaces 2014, 1:1300138].
In many industrial and scientific applications, the hydrophobic nature of liquid substances such as oils and hydrophobic liquid polymers is undesirable and methods of increasing the wettability of these liquids are extensively sought for.
The degree of hydrophilicity of a surface is commonly measured by contact angle symmetry. The contact angle (θ) is the angle at which the subject liquid (e.g., water) interfaces the surface and is determined by the adhesive and cohesive forces of the liquid. As the tendency of a drop to spread out over a surface increases, the contact angle decreases and vice-versa. Thus, the contact angle provides an inverse measure of hydrophilicity or wettability. As a general rule, a surface exhibiting high wettability will have a small contact angles (e.g., smaller than 90°) while a large contact angles (e.g., larger than 90°) is indicative of a non-wettable surface. Due to the important role surface hydrophilicity plays in industrial and research processes, numerous procedures and methods have been developed to modify the wettability of a surface and achieve the desired properties.
One such procedure is plasma treatment. Plasma is a state of ionized gas, sometimes referred as “fourth-state-of-matter”, consisting of reactive particles such as electrons, ions and radicals. Plasma treatment (low-pressure and atmospheric-pressure) is widely used for the modification of surface properties of solid polymer materials. The plasma treatment creates a complex mixture of surface functionalities which influence surface physical and chemical properties and results in a dramatic change in the wetting behavior of the surface.
Plasma treatment usually strengthens the hydrophilicity of treated solid polymer surfaces. However, the surface hydrophilicity created by plasma treatment is often lost over time. This effect of decreasing hydrophilicity is called “hydrophobic recovery”, and is usually attributed to a variety of physical and chemical processes, including: (1) rearrangement of chemical groups of the surface exposed to plasma treatment, due to the conformational mobility of polymer chains; (2) oxidation and degradation reactions at the plasma-treated surfaces; (3) diffusion of low molecular weight products from the outer layers into the bulk of the polymer; and (4) plasma-treatment induced diffusion of additives introduced into the polymer from its bulk towards its surface [Pascual et al., J Mater Sci 2008, 43:4901-4909].
Plasma treatment of solid polydimethylsiloxane surfaces has been reported to enhance hydrophilicity, and to be followed by gradual hydrophobic recovery [Owen & Smith, J Adhes Sci Technol 1994, 8:1063-1075; Morra et al., J Colloid Interface Sci 1990, 137:11-24].
U.S. Pat. No. 4,822,632 teaches plasma treatment of surfaces covered with PDMS oils were used for the purpose of reducing friction between moving parts. U.S. Pat. No. 6,461,334 teaches surfaces coated by a hydrophilic coating, which can be achieved, inter alia, by plasma treatment of the coating. The treated coating is a solid coating or a coating which becomes solid upon treatment.
Additional background art includes plasma treatment of solid polymers for surface modification and adhesion improvement [Dirk Hegemann et. al, Nuclear Instruments and Methods in Physics Research, 2003, B208, 281-286]; Hydrophilization of liquid surfaces by plasma treatment [E. Bormashenko et al., Colloids and Surfaces A: Physicochem. Eng. Aspects 461 (2014) 225-230]; and Quantification and Physics of Cold Plasma Treatment of Organic Liquid Surfaces [E. Bormashenko et al., eprint arXiv:1503.00425, published March 2015].